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Posted By: fostertomPlenty of evidence that that situation causes saturation of the timber, even if the overall humidity levels are nothing special.
Posted By: CWattersI thought trickle vents provided as much as 3 air changes an hour
Posted By: fostertomI'll see if I can - it was a Canadian study on this subject, Grd Fl over 'crawl space', that you posted, that first alerted me. Do you remember what that was?
Posted By: fostertomThis is really poor stuff.
Posted By: skywalkerI remember the 'crawlspaces' document well
Posted By: skywalkerI remember the 'crawlspaces' document well. It provides a hugely positive endorsement for the use of PUR under floors
Posted By: skywalkerThe Article may set the scenario in summer but the issue is the same as here higer temp & humidity air from the inside meeting colder saturated air from the outside.From the article:"The average dew point of the exterior air during the summer months in Washington, DC is 65 degrees. Let’s bring this air into the crawlspace—so therefore the dew point of the air in the crawlspace is 65 degrees. Recall that the top of the floor joist is 75 degrees. The floor joist sees the dew point of the air in the crawlspace (we can ignore the vapor permeance characteristics of the fiberglass batt insulation since it is so vapor open—just pretend that we have air rather than insulation here—but not just any air—air with a huge temperature drop— “insulating air”), but because the floor joist is 75 degrees at this location, the relative humidity at this location is 70 percent yielding a wood equilibrium moisture content of 13 percent. The floor joist is “dry” at the top and “wet” at the bottom. Why wet at the bottom? The surface of the wood is cold, below the dew point of the air in the crawlspace and therefore condensation forms on the wood. At fiber saturation the moisture content of wood is 28 percent (go to Figure 3 again).'So the vapour barrier in the solution is on the cold side.S.
Posted By: skywalkerI haven't seen a construction type like this advocated by any PUR manufacturer. Do you know of any Tom?Warm roof upside down?S.
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